Silicon Carbide Sensor Systems for Harsh Environment Market Applications M. Andersson1,2, B. Hammarlund1, A. Lloyd Spetz2, D. Puglisi2 1 SenSiC AB, Isafjordsgatan 39 B, SE-16440 Kista, Sweden 2 Div. Applied Sensor Science, Dept. Physics, Chemistry and Biology, Linköping University, SE-58183 Linköping, Sweden [email protected] Abstract The silicon carbide (SiC) processing technology for 4 inch wafers, with 6 inch in the planning phase, has reached matureness of mass production. Field effect devices based on silicon carbide, SiC-FET, employing a catalytic metal such as porous iridium (Ir) as the gate material change the current-voltage characteristics due to gas interactions with the gate metallization. Several batches of sensor devices have improved the device performance and gas response characteristics and, together with important developments on packaging of the sensors, electronics and software, sensor systems are nowadays commercially available. Here we report on Ir-gated SiC-FETs for indoor and outdoor air quality monitoring and control applications. In particular, we focus on the highest sensitivity of such sensors to ultra-low concentrations of three hazardous volatile organic compounds, i.e., benzene, naphthalene, and formaldehyde, which are commonly found in indoor environments and are considered carcinogenic already at the low parts per billion concentration range. Key words: Silicon carbide, field effect transistor, gas sensor, air quality monitoring, ppb detection, indoor/outdoor environment Introduction developed for control of the injection of urea in Silicon carbide based field effect transistor stationary engines with selective catalytic (SiC-FET) technology has been developed over reduction (SCR) to reduce nitrogen oxide gases the last 20 years, showing very good from diesel exhausts [3]. A sensor system with possibilities of realizing gas sensors for high the possibility to detect the ratio between CO temperature and harsh environment and oxygen is commercialized for control of applications such as combustion monitoring small and medium sized bioheaters [4]. A SiC and control of car exhaust after-treatment based oxygen sensor is under development to systems [1,2]. Today, such technology has control exhaust gas recirculation (EGR) in car become viable for the commercial production of engines. Another SiC sensor device in research high volume, and price competitive products. is devoted to detection of nitrogen oxide gases SiC based gas sensors offer full functionality in in car exhausts. a wide temperature range, harsh environments, A 4 inch SiC wafer with transistors for gas and good resistance to thermal shock. A single sensors, designed by SenSiC AB and sensor unit can detect several gas molecules, processed at Ascatron AB, Sweden, is shown in such as carbon monoxide (CO), ammonia Fig. 1. The chips are mounted in a holder (NH3), hydrogen (H2), and hydrocarbons (HC) designed to withstand harsh environment and like certain volatile organic compounds (VOCs), vibration. The electronics box is also displayed. depending on the working temperature and on the sensing material used for the gate contact. Together with such applications for emissions SiC processing technology for 4 inch wafers, and outdoor air pollution monitoring, we have with 6 inch in the planning phase, has reached developed SiC-FET gas sensors for indoor air matureness of mass production. The silicon quality [5,6]. Recently, Ir-gate SiC-FETs have carbide offer a very robust substrate platform, been used for highly sensitive and selective which even at an operating temperature over detection of low parts per billion (ppb) 500 °C can withstand sprinkling by water concentrations of three target VOCs, i.e., droplets, which occur e.g. in a diesel engine formaldehyde (CH2O), naphthalene (C10H8), compartment. An ammonia sensor has been and benzene (C6H6). The sensor performance Fig. 1. A 4 inch SiC wafer with transistors for gas sensors, designed by SenSiC AB, processed at Ascatron AB. The chips are mounted in a holder designed to withstand harsh environment and vibration. The electronic box is also displayed. and characteristics were investigated under measuring the drain current, ID. A separate gate controlled lab conditions at different levels of voltage, VGS, was applied using a stabilized relative humidity (r.h.) up to 60 %, showing voltage source. I-V characteristics were good stability and repeatability of the sensor measured at zero and 2 V of applied VGS (see response, and detection limits in the sub-ppb Fig. 3). As can be seen from Fig. 3, the range [6]. saturation current decreases with increasing temperature, and increases with increasing The SiC-FET technology can therefore be used gate voltage. The saturation region starts for both outdoor and indoor air pollution between 0.5 V and 1.5 V depending on the monitoring and control applications. applied gate bias and operating temperature. It has been demonstrated that, despite that the SiC-FET gas sensor fabrication value of the saturation current differs from Two- and three-terminal depletion type metal insulator semiconductor field effect transistor VDS V V DS GS (MISFET) devices were fabricated on top of 4 VDS I VVGS inch n-type 4H-SiC wafers. Nanostructured D GS porous Ir, used as the catalytic metal-gate D S contact, was deposited by DC magnetron n-type activeD layer S sputtering at an Ar pressure of 50 mTorr to a n-type active layer p-type buffer layer total thickness of 30 nm. The gate width is 300 p-type buffer layer µm and the corresponding gate length is 10 µm, n-type 4H-SiC substrate with a separation between the gate and the n-type 4H-SiC substrate source-drain contacts of 5 µm [1]. The sensor chip, 2 mm × 2 mm sized and containing four SiC-FET devices, was attached to a ceramic heater substrate (Heraeus PT 6.8/1020) (a) together with a Pt100 temperature sensor, and mounted on a 16-pin TO8 header (see Fig. 2). Electrical connections were made using gold wire bonding. Electrical characterization Current-voltage (I-V) measurements were carried out at 27, 100, 200, and 300 °C on several three-terminal depletion type SiC-FETs with iridium gate. A source meter Keithley 2601 (b) was used to operate the devices sweeping the Fig. 2. (a) Cross sectional view of a three- voltage VDS over the drain-to-source contacts terminal SiC-FET and (b) top view of a SiC-FET from 0 to 5 V at a rate of 0.1 V/s, and sensor chip mounted on a 16-pin TO8 header. exhibits a very low electronic noise. The relative 27°C response is 1.25 % at 4 ppb (sensitivity 1.75 Ir-gated SiC-FET 1000 100°C mV/ppb) with a relative error of 0.14 %. The 200°C response time is about 3 min, whereas the 27°C A) 800 100°C 300°C recovery time is about 6 min. Measurements Straight line: V = 2 V GS V 200°C were repeated several times at different 600 GS 300°C Dash line: VGS = 0 environmental conditions for a total duration of 400 two weeks. The presence of high humidity Drain Current ( levels does not significantly affect the sensor 200 performance with respect to the parameters 0 studied. As previously reported [6], the 0 1 2 3 4 5 detection limits at 60 % r.h. for benzene (C6H6), Drain-to-Source Voltage (V) naphthalene (C10H8), and formaldehyde (CH2O) Fig. 3. Current-voltage characteristics of a three- are in the range of about 1 ppb, 0.5 ppb, and 10 terminal depletion type Ir-gate SiC-FET from room ppb, respectively. These results are very temperature to 300 °C at zero and 2 V applied gate promising for indoor air quality applications. The voltage. detection limits of CH2O and C10H8 are below the threshold limits recommended by the World sample to sample, it does not significantly affect Health Organization (WHO), being 81 ppb (30 the sensing behavior [1,7]. min exposure) for CH2O and 1.9 ppb (annual average) for C10H8 [9]. The legislation around VOC tests C6H6 is even stricter, since no safe level of Ir-gated SiC-FETs were operated at three exposure are recognized for benzene [9]. different constant temperatures, i.e., 260, 300, Detecting levels of this VOC down to about 1 and 330 °C, in dry air and at different levels of ppb in presence of high humidity levels is for relative humidity (r.h.) from 10 to 60 %. sure an encouraging result. Benzene, naphthalene, and formaldehyde were Further investigation includes the development used as target gases at several concentrations of new sensing materials to be used as the gate down to the sub-ppb level in order to study the contact. Recently, we have combined the sensor’s sensitivity, stability, repeatability, and deposition of porous Ir with thin films of speed of response. The sensor devices were tungsten trioxide (WO3) using pulsed laser operated at the onset of the saturation region, deposition (PLD) process in collaboration with keeping the drain current constant, ID, and the University of Oulu, Finland. From the gate bias, VGS, and measuring the drain-to- combination of a catalytic metal and metal source voltage, VDS, as the sensor output. oxide we expect to enhance the sensor performance especially in terms of selectivity Results and discussion and stability during long term operation. A complete data analysis of the three studied Preliminary results are currently under VOCs at different temperatures, levels of evaluation. relative humidity, and gas concentrations can be found elsewhere [6,8]. Figure 4 shows the Conclusions sensor response to benzene at 330 °C and 60 SiC-FET sensor technology has demonstrated % relative humidity (r.h.). The sensor signal over the last 20 years to be suitable for both outdoor and indoor air quality monitoring and Ir-gated SiC-FET control applications.